Dual pinion variable speed drive systems are known to be used in cement mills where two variable speed drives are connected to the same cement mill. An example of such a system 2 is shown in FIG. 1 and comprises a first motor 11a, a second motor 11b, corresponding drive convertors 15a and 15b, and a cement mill 16.
The components of such a system feature a number of physical properties such as inertia, stiffness and damping and during its operation there is interplay of these properties between each component within the system. An illustration of the interplay of mechanical properties between the components is shown in FIG. 2. Motor components 11a, b act upon load component, in this case a cement mill, 16 via coupling in components 12a, b and coupling out components 14a, b. Each of the components 11a, 11b, 12a, 12b, 14a, 14b and 16 represents an inertia within the system. Torque 10a, 10b is input into drives 11a, 11b respectively and between each of the motors 11a and coupling in component 12a, coupling in component 12a and coupling out component 14a, coupling out component 14a and load 16 there is acting a mechanical stiffness 13a and mechanical damping 13b. Output from the load is the load torque 18a as well as mechanical damping 13b. 
As a result of the interplay of these mechanical properties within the system, a dual pinion drive system having variable speed drives can be prone to issues caused by torsional oscillations. If the system is considered as a three mass system, namely motors 11a, 11b and load 16, the dual pinion drive system can exhibit two natural modes of low frequency oscillation. In the first mode, motors 11a and 11b oscillate in counter phase wherein motor 11a speeds up whilst motor 11b slows down and vice versa. For example, motors 11a and 11b alternately pull the load, which maintains a constant speed. The natural frequency of systems, having soft couplings, operating in this mode has been observed to be about 1 Hz to 5 Hz. In the second mode, motors 11a and 11b oscillate in phase with each other but counter to the load 18. The natural frequency of a system operating in this mode has been observed to be about 3 Hz to 8 Hz, which can be higher than the natural frequency of the first mode.
In view of the variation in the natural oscillation frequency of systems operating in the first mode or the second mode, the frequency response aspect of their design takes into account the inclusion of controls capable of damping both of the modes of natural oscillation.
The frequency response of such a known dual pinion drive system, when used for example in a Semi Autogenous Grinding Mill, is shown in FIG. 3. The graph, line A in the graph illustrates the comparison of the torque reference of the first motor 11a to the rotational speed of the motor 11a and this shows the counter-phase and in-phase oscillations of the frequency response. Also provided for the sake of comparison is the frequency response in a single pinion configuration, shown by line B. As shown in from Line A, in addition to the natural modes of oscillation for a three mass system, a high frequency mode can also be seen around 240 Hz. This high frequency mode can be as a result of a flexible coupling in the drive train model. The illustration in the graph primarily shows that a source of oscillations in the torque reference in the drive train has the potential risk of being amplified at three different frequencies. Therefore, it may be important that the control system is suitably assessed to establish the effectiveness in damping these oscillations.
In a dual pinion variable speed drive system such as that of FIG. 1, the motors 11a, 11b are controlled using a master-follower configuration wherein one of the motors, 11a, is the master drive and is responsible for speed control of the system by virtue of providing input into a common Proportional Integral (PI) speed controller meaning motor 11b is the follower drive. The rotational speed of the master drive is input into the PI speed controller where it is compared against a predetermined set-point to calculate a torque reference signal. From this, a torque command is generated by the PI speed controller and is sent to both the master drive converter 15a and the second drive converter 15b. The converters 15a and 15b using their existing internal control system (not shown), can then produce the necessary voltage waveforms to drive their respective motors 11a, 11b according to the generated torque commands. An illustration of this process is show in FIG. 4.
Regardless of the PI speed controller tuning parameters, the counter phase oscillations are not damped and thus can pose a potential risk to the system. FIG. 5 shows the frequency response of the motor speed of the follower drive as influenced by the speed reference changes in the master drive motor. The nominal setpoint tracking performance from the control system can cause significant oscillations in the motor speed of the follower drive and thus the torque load sustained by the shafts interconnecting the components of the system.
Detuning the control settings of the PI speed controller reduces the magnitude of the oscillations, however as a result of the detuning the control performance degrades beyond acceptable limits for the system.
Another approach to overcome these issues has involved carrying out extensive low-pass filtering of the feedback signals. The low pass filtering can damp the oscillations within the system, however it can slow the overall closed-loop dynamics of the system.